This invention relates generally to a sealing member and the method of fabricating the same and, more particularly, to a sealing member employed in cryogenic atmospheres to prevent leakage of flowable cryogenic fluids.
While the sealing member of this invention will conveniently be described in connection with the cooling system employed to maintain an imaging infrared detector at cryogenic temperatures in an autonomously guided missile, it should be appreciated that the sealing member of this invention is in no way restricted to such usage, but has utility in a wide variety of applications where it is desirable to trap or otherwise preclude leakage of fluids ranging from room temperatures to cryogenic temperatures.
Certain missiles utilize imaging infrared (IR) guidance to provide a day or night capability under a wide variety of target conditions. The infrared detector is effective to form a display image by sensing the differences in infrared heat radiated by objects in view for homing in on selected targets. In order to perform satisfactorily, the detector which is mounted in a dewar, must be maintained at cryogenic temperatures in order to properly sense minute differences in temperature. The component relied upon to transmit extreme low temperatures is a cryocooler expander element commonly referred to as a "cold finger". The cold finger is cylindrical in shape and is closely fitted within the double-walled dewar surrounding the cold finger. These components are very small, the cold finger diameter ranging between about 0.194 to 0.196 inches and the inside diameter of the dewar ranging from about 0.2005 to 0.2023 inches, leaving a gap therebetween of approximately 0.0055 to 0.00725 inches, depending on the exact dimensions of the acceptable tolerances between these components. This small gap is exposed to the ambient atmosphere consisting of a suitable cryogenic gas, such as nitrogen for example, which liquifies at a given Kelvin temperature as the cold finger cools. The resultant liquid nitrogen resides between the distal or cold end of the cold finger and the bight portion of the dewar section in which the detector is mounted providing improved conduction and cooling therebetween.
A serious problem resides in maintaining the liquid nitrogen in place during the extreme acceleration forces encountered when the missile is launched. Leakage of the liquid nitrogen through the gap from the area adjacent the dewar detector interface destroys the thermal conduction effective thereon to raise the temperature of the detector and causes it to go blind.
One approach in solving this problem was to fill the dewar/cold finger gap with plastic tape. The installation procedure included stretching the tape before application to reduce its thickness and then applying it over the last one third of the cold end of the finger. The cold finger was then inserted into the dewar, allowing the tape to relax and expand in thickness to fill the gap. However, it was found that the plastic tape became very hard at cryogenic temperatures, creating a rigid interface resulting in binding and the transmission of undesirable forces between the cold finger and the dewar.